With time axes you cannot specify annotation intervals of 500 seconds. That is 8 minutes and 20 seconds. Is that what you really want? Your time axis represents 10 days so that would be 10*86400/500 = 1728 annotations, all unreadable of course.Basically, use the units if you want more than 60 seconds, e.g. 10m for 10 minutes. The error is that you asked for more than 60 seconds, which means you should use minutes or even hours.